lxcbtrfs.{c,h} rework declarations and definitions

lxcbtrfs.{c,h} rework declarations and definitions

Declare

	- btrfs_same_fs();
	- btrfs_snapshot();

extern instead of static in lxcbtrfs.h. They are defined in lxcbtrfs.c.

Forward declare/put

	- struct bdev; /* defined in bdev.h */
	- struct bdev_specs; /* defined in lxccontainer.h */
	- struct lxc_conf; /* defined conf.h */

as incomplete types in lxcbtrfs.h so that functions declared and defined in
lxcbtrfs.{c,h} have access to it.

Declare

	- dir_new_path();

in lxcbtrfs.c. It is defined in lxccontainer.c.

Move definition of struct

	- struct rsync_data_char;

from bdev.c to bdev.h because the functions in lxcbtrfs.{c,h} need to access it.

Signed-off-by: Christian Brauner <christian.brauner@mailbox.org>